The heart includes a number of pathways through which electrical signals necessary for normal, electrical and mechanical synchronous function or the upper and lower heart chambers propagate. Tachycardia, that is abnormally rapid rhythms of the heart, are caused by the presence of an arrhythmogenic site or accessory pathway which bypasses or short circuits the nodal pathways in the heart. Tachycardias may be categorized as ventricular tachycardias (VTs) or supraventricular tachycardias (SVTs). The most common SVT's include atrioventricular nodal reentrant tachycardia (AVNRT), Atrioventricular reentrant tachycardia (AVRT), atrial fibrillation (AF), and atrial flutter (AFI). Reentrant tachycardias originate in the atria and are typically caused by an accessory pathway or inappropriate premature return excitation from the ventricle through the AV node or left sided accessory pathway. Conditions such as AF and AFI involve either premature excitation from focal ectopic sites within the atria or excitations coming through inter-atrial reentry pathways as well as regions of slow conduction within the atria. VT's originate from within the ventricles and have their entire circuit contained within the ventricles. These VT's include bundle branch reentrant tachycardia (BBR), right ventricular outflow tract tachycardia (RVOT), and ventricular fibrillation (VF). VT's are often caused by arrhythmogenic sites associated with a prior myocardial infarction as well as reentrant pathways between the ventricles. BBR involves an inappropriate conduction circuit that uses the right and left bundle branches. RVOT can be described as a tachycardia originating from the right ventricular outflow tract which involves ectopic triggering or reentry mechanisms. VF is a life threatening condition where the ventricles entertain a continuous uncoordinated series of contractions that cause a cessation of blood flow from the heart. If normal sinus rhythm is not restored, the condition is terminal.
Treatment of both SVTs and VTs may be accomplished by a variety of approaches, including drugs, surgery, implantable electrical stimulators, and catheter ablation of cardiac tissue of an effected pathway. While drugs may be the treatment of choice for many patients, drugs typically only mask the symptoms and do not cure the underlying cause. Implantable electrical stimulators, e.g., pacemakers, afferant nerve stimulators and cardioverter/defibrillators, usually can only correct an arrhythmia after it occurs and is successfully detected. Surgical and catheter-based treatments, in contrast, will actually cure the problem usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. The catheter-based treatments rely on the application of various destructive energy sources to the target tissue including direct current electrical energy, radio frequency (RF) electrical energy, laser energy, ultrasound, microwaves, and the like.
RF ablation protocols have proven to be highly effective in treatment of many cardiac arrhythmias while exposing the patient to minimum side effects and risks. RF catheter ablation is generally performed after an initial electrophysiologic (EP) mapping procedure is conducted using an EP mapping catheter to locate the arrhythmogenic sites and accessory pathways. After EP mapping, an RF ablation catheter having a suitable electrode is introduced to the appropriate heart chamber and manipulated so that the electrode lies proximate the target tissue. Such catheters designed for mapping and ablation, frequently include one or more cylindrical or band-shaped individual electrodes mounted to the distal section of the catheter so as to facilitate mapping of a wider area in less time, or to improve access to target sites for ablation. RF energy is then applied through the electrode(s) to the cardiac tissue to ablate a region of the tissue that forms part of the arrhythmogenic site or the accessory pathway.
Ablation of VT's can be difficult due to the thickness of the ventricular chamber walls. Typical RF delivery through standard electrodes is not capable of creating deep transmural lesions in the ventricles. When RF power is raised to high levels, tissue charring and subsurface steam explosions can occur. Coagulum buildup on the electrode surfaces leads to high impedance problems and more importantly, thrombi may be released that could cause stroke. These factors present major problems that limit the safe depth to which lesions can be created. To overcome these problems, saline irrigated electrodes were developed to allow more efficient RF delivery to the myocardium. These irrigated systems nearly eliminate coagulum buildup that would cause impedance rises and increase the risk of stroke. Irrigation keeps the metallic electrodes cool which prevents endocardial surface charring and tissue dessication. With irrigated RF ablation, there remains the problem of creating excessive subsurface temperatures that can lead to steam explosions and cratering of the endocardium.
The following remarks generally apply to catheters designed to perform either one or both of the EP mapping and RF ablation functions, unless otherwise expressly indicated. Illustrative catheters of this type are described in commonly assigned U.S. Pat. Nos. 5,318,525, 5,545,200 and 5,823,955, for example. As described therein, it is frequently desirable to deflect a distal tip section of the catheter body into a non-linear configuration such as a semicircle or curved configuration, which facilitates access to the endocardial heart wall to be mapped or ablated. Such deflection may be accomplished through the use of pull wires secured along the distal tip section which can be tensioned by a control on the handle at the proximal end of the catheter to deflect the tip in the desired configuration. In addition, rotational positioning of the distal tip section is accomplished, either by rotating the entire catheter from the proximal end, or by exerting torque on a core wire secured to the distal tip without rotating the catheter body itself as disclosed in the above-referenced '525 patent. Moreover, selectively retractable stiffening or deflecting core wires are also employed in the design of such catheters as shown in the above-referenced '200 patent for example.
Such mapping and ablation catheters are inserted into a major vein or artery, usually in the neck or groin area, and guided into the chambers of the heart by appropriate manipulation through the vein or artery. The catheter must have a great deal of flexibility or steerability to be advanced through the vascular system into a chamber of the heart, and the catheter must permit user manipulation of the tip even when the catheter body traverses a curved and twisted vascular access pathway. Such catheters must facilitate manipulation of the distal tip so that the distal electrode(s) can be positioned and held against the tissue region to be mapped or ablated.
While EP mapping and RF ablation catheters having the aforementioned deflectability and steerability have had promising results, such catheters suffer from certain disadvantages. The catheters disclosed in the '200 patent provide a continuous curve of the distal tip section having a selectable radius so that the plurality of ring-shaped electrodes are distributed in a desired curved to bear against the heart wall at certain sites. The above-referenced, commonly assigned '200 and '955 patents have at least two segments in the distal tip section of the catheter body that are independently variable. The '955 patent discloses a curvature of the proximal segment of the distal section in one direction, and the distal segment of the distal section in the opposite direction but in the same plane as the proximal segment. The '955 patent distal tip section configuration is particularly adapted for mapping and ablation of tissues around the right and left heart atrioventricular (AV) valve annulus. The '200 patent also discloses a curvature of the distal segment of the distal section in a lateral direction, put of the plane of the curvature established independently in the proximal segment of the distal section. The degree of deflection of the distal segment with respect to the proximal segment is limited, and the curves that can be obtained in the distal segment are limited. Moreover, the limited curvature or angular displacement of the distal segment with respect to the proximal segment and the proximal section of the catheter body does not make it possible to optimally apply the distal tip electrode(s) against other target points or sites of the heart wall or endocardium.
A steerable catheter for mapping and/or ablation is needed that enables mapping and ablation about a variety of structures of the heart comprising particularly about various vascular orifices or valves entering the right and left atria and the valves between the atria and ventricles.
Furthermore, there is a need for a catheter having the capability of abruptly changing the angle of the tip electrode(s) bearing segment with respect to the more proximal catheter shaft in order to enable full length tissue contact of the side of an elongated electrode or set of electrodes with the heart tissue to be mapped or ablated.